Here's my own take on Horrortale
The King and Queen are both gone. The human SOULS have vanished. With no one else to step up to the throne, Undyne takes it. Making the announcement that they will get revenge for every horrible deed done to them. Buckling down on keeping the children safe, getting everyone fed, and keeping the areas safe and secure. The edge of the waterfalls in Waterfall are closed off so no one can accidentally fall down into the abyss there. Any open bridges are now required to have security in place, most having just handrails but some having cage tunnels helping. Secure roads through Hotland are made deeming travel by steamvents unnessasary and lasers taken down in place of more guards so there are no accidents. The True Lab, once discovered, is put on lock down. The Amalgamates returned to their families as Alphys is nowhere to be found.
As time goes on, their rage and anger fills them, increasing their magic and turning them into more terrifying versions of themselves. Undyne, both with the increase of power and the sadness of losing both the King and Queen to the human, and Alphys disappearing, gets into blind rage at times. Papyrus forced himself into the role of Head of the Guard to watch after her which she thanks him for. As for Sans, he had turned even more reclusive than before. He doesn't remember the Resets fully, but knows that the kid could come back and fix all of the mess they caused. And each day he also feels more and more regret keeping the promise to protect the human. At Papyrus' request, Undyne keeps him busy with light scouting missions to get more space for the Kingdom.
Which works for a while, but then one day he was lost in thought, trying to remember something, when a stalagmite falls, cracking open his head, leaving him immobile for a while until Papyrus comes to find him. He holds onto the rage at himself for letting the kid go, and at the kid for not coming back and leaving them all to dust in the stars forsaken mountain they were forced to call home. He's still mumbling about how it was their fault when Papyrus found him and got him to emergency care. He wanted some way to comfort Sans but couldn't find the reason to refuse. Papyrus blamed himself for letting the human go instead of bringing them to Undyne when he had the chance, promising himself to always protect Sans.
With these new outlooks on humans, they grow, stronger and bigger. One other thing is that they change their names to leave their old lives and hopes behind. Papyrus names himself Dritch and Sans calls himself Dread. They fight together, protecting each other's weaknesses: If you get on their bad side there's a high risk you won't make it out again. Threaten them, threaten to hurt the other, their friends, everything that falls into that catagory. As long as they protect their people, they don't care for gaining LOVE or EXP.
A few decades later, they finally break the Barrier with enough SOULS. But at this point, the bloodlust was turned to self preservation. So, Queen Undyne orders an attack, soon taking over quite a large area of land for the monsters to settle into. Making their new kingdom on the Surface, refusing any humans to step foot on their land ever again.
While they still work under Undyne, Dritch and Dread have also taken up hobbies of their own. Dritch works on making weapons and defense items, just in case any human tries to disable their magics, while Dread helps teach younger monsters to fight.
Together they fight to protect their kingdom, the Monster's Den.